This was absolutely amazing and totally underrated! Find my video review:
00:50 Pygoscelis
Thank you for the review Bastian! I had a blast watching you play the game :D
The color change in a simple transition shader, the same as for a circular in-out black screen when you go through a door in many games.
Then I just give this shader a texture that's made with a watercolor brush, and ta-da ! :D As simple as that